1. Field of the Invention
This invention relates to electrical devices comprising a resistive element and a connection element attached thereto.
2. Introduction to the Invention
Many electrical devices comprise a resistive element and at least one connection element, the connection element comprising (a) a first portion which is directly attached to, e.g. embedded in, the resistive element, and (b) a second portion which extends outwards from the resistive element and which is connected to the remainder of the circuit. Usually there are two such connection elements of identical characteristics. A number of methods have been used, or proposed, for manufacturing such devices. These methods include processes in which the resistive element is formed by shaping a suitable material into a continuous strip or sheet, and then cutting the strip or sheet into discrete elements. In one such method, the connection element is attached to the resistive material after it has been shaped, either before or after the shaped resistive material is cut into discrete elements. In another method, the resistive material is shaped around an elongate preconnection element, e.g. by extruding a conductive polymer over a pair of wires; the extrudate is cut into discrete lengths; and a part of the resistive material is removed so as to expose the connection element. In another method, the resistive material is shaped against one or more preconnection elements, e.g. a conductive polymer is laminated as a sheet between two metal foils; the resulting product is cut into discrete parts, and leads (which become the second portion of the connection element) are secured to the exposed parts of the connection elements. In another method, the resistive material is shaped against one or more preconnection elements which extend outwardly from the shaped material; and the resulting product is cut into discrete parts, with the connection elements extending from the resistive material. Reference may be made, for example, to U.S. Pat. Nos. 3,351,882 (Kohler et al), 4,238,812 (Middleman et al, 4,327,351 (Walker), 4,352,083 (Middleman et al), 4,413,301 (Middleman et al), 4,426,633 (Taylor), 4,445,026 (Walker), 4,481,498 (McTavish et al), 4,685,025 (Carlomagno), 4,689,475 (Matthiesen), and 4,800,253 (Kleiner et al), the disclosures of which are incorporated herein by reference.
All of the methods referred to above suffer from serious problems, for example, one or more of: failure of economically attractive processes to provide good contact between the resistive material and the connection element; failure of economically attractive processes to provide good contact between the lead and the first portion of the connection element; failure of the second portion of the connection element to have required properties for connection to other parts of a circuit, e.g. adequate rigidity for insertion into a printed circuit board; and undesirable effects of the connection element on the properties of the resistive element, e.g. excessive physical restriction of a PTC conductive polymer resistive element.
One type of electrical device of particular interest is a circuit protection device in which the resistive element comprises a conductive polymer. Such devices, which exhibit positive temperature coefficient (PTC) behavior, are particularly suitable for providing protection against overcurrent or over-temperature faults in an electrical circuit. Under normal conditions, the device has a low resistance which allows the normal current to flow in the circuit. If, however, the device is exposed to a high ambient temperature or experiences joule heating resulting from a fault current (e.g. a voltage spike), the resistance of the device increases and interrupts the current flow. When the fault condition is removed, the device cools down, the resistance drops, and the normal circuit operation resumes. When the device is in its high resistance state, it is said to have "switched" or "tripped". The "switching temperature", T.sub.s, is used herein to denote the temperature at the intersection point of extensions of the substantially straight portions of a plot of the log resistance of the device as a function of temperature which lie on either side of the portion showing a sharp change in slope.
Electrical connection to the circuit protection device is made by means of connection elements which are electrodes, i.e. electrically conductive leads or busbars which are electrically attached to the PTC element which comprises the conductive polymer. When it is desired that the circuit protection device be machine-insertable into a circuit board, it is preferred that at least a portion of the electrode be solid, rather than stranded, wire. Solid wire of a given diameter is generally stiffer than stranded wire of the same diameter, a feature which aids insertion into a hole on a board. In addition, solid wire is not subject to inconsistent dimensions resulting from nonuniform stranding, nor is it subject to unravelling strands or "birdcaging", i.e. the unstranding of wire which occurs when pressure is applied nonuniformly to the end of a stranded wire. Solid wire, however, may be so rigid that the expansion of the conductive polymer in the PTC element during tripping may be restricted, resulting in device failure; the wire may not "give" enough to survive repeated electrical cycles, particularly at high voltages, e.g. greater than 120 volts. In addition, when compared to a stranded wire, the surface area of a solid wire may not be large enough to allow adequate adhesion of the conductive polymer to the electrode. The resulting device will thus have areas of poor contact to the electrode; the contact will deteriorate with each cycle, resulting in eventual device failure.